This invention relates, in general, to radioactivity well logging, and more particularly to methods and apparatus for determining the macroscopic thermal neutron absorption cross-section of the formations surrounding a borehole as determined by radiation measurements.
It is well known to make a log of the various earth formations traversed by a borehole by passing a source of high energy neutrons through the borehole to bombard the surrounding earth and by accompanying the neutron source with a radiation detector which is responsive to radiation resulting from such bombardment. The logging instrument is composed of an elongated pressure-resistant housing containing the neutron source which is usually located adjacent one end and one or more radiation detectors spaced in the housing at a preselected distance from the source. The detector is preferably adapted to generate electrical pulses corresponding to detected radiations, these electrical pulses being coupled to the cable for transmission to electronics measuring equipment located of the surface of the earth.
When a neutron emanates from the source, it interacts with the materials present in the borehole and formation and are slowed down. After the neutrons are thermalized, they are captured by atoms in the earth formations placing those atoms in an excited state, gamma radiations are emitted and the atoms return to a stable state. The "capture" gamma rays are detected by the detector in the logging instrument.
It is well known in the art of radioactivity well logging, for example, as illustrated and described in U.S. Pat. Nos. 3,379,882 and 3,379,884 which issued to Arthur H. Youmans and each of which is assigned to the assignee of the present invention, to measure the macroscopic thermal neutron capture cross-section [Sigma (.SIGMA.)] of the formations surrounding a borehole. This prior art method makes such a measurement or computation by measuring the decline of the thermal neutron population by measuring the decline of the thermal neutron population in such formations within a fixed period of time following the emission of a burst of high energy neutrons and by dividing the radiations indicative of such thermal neutrons into two equal groups and computing the rate of change over the selected time interval.
It has been discovered, however, that when using two measurement intervals of fixed periods following the neutron burst the resulting Sigma measurement can have unacceptable statistical variation when the instrument encounters formations of high Sigma value.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing method and apparatus for measuring the macroscopic thermal neutron capture cross-section using optimized measurement intervals.